Where organic compounds, particularly polymers and oligomers, are used as an electronic device material, they are frequently employed as a thin film. For instance, mention is made of an insulating film, charge transporting film, protective film and flattened film.
In organic electroluminescent (hereinafter abbreviated as EL) devices, charge transporting thin films made of polymers or oligomers are used as a hole transport layer (buffer layer) and a charge injection layer (see, for example, Patent Document 1).
It is required that the charge transporting thin film can be formed on a flat electrode such as of indium tin oxide (hereinafter abbreviated as ITO), indium zinc oxide (hereinafter abbreviated as IZO) or the like in the form of a uniform, unevenness-free thin film. However, when a charge transporting thin film is formed actually in an electronic device, a structure is formed on the electrode in most cases. More particularly, a structure of at least one simple or complicated pattern having a thickness of several μm may be formed in some case. A charge transporting varnish of a type, which is used to selectively, flatly form a charge transporting thin film on an exposed ITO or IZO electrode portion of a substrate on which a structure constituted of a simple or complicated pattern has been formed, may frequently differ in liquid physical properties from a charge transporting varnish of the type used to form a charge transporting thin film uniformly on a flat electrode. Thus, the liquid physical properties of a charge transporting varnish adapted for the structure on a substrate need to be studied in detail.
Among electronic devices, on the other hand, an organic EL device has recently drawn attention as a next-generation thin display that is being put to practice, and is one of electronic devices, which is now under study for practical use. As it becomes more likely for the device to come into practical use, it has been required in the production line to make a highly reliable device inexpensively and in good yield and high efficiency. Especially, the inexpensiveness is a self-evident requirement, with a tendency to meet the requirement through a trial and error approach to processes of making a charge transporting thin film.
The process of making a charge transporting thin film can be broadly classified into a dry process, typical of which is a vacuum deposition process, and a wet process typified by a spin coating process. According to the comparison between the dry process and the wet process, the charge transporting thin film formed by the wet process rather than the dry process is higher in the ability of covering an electrode such as of ITO or IZO and is able to uniformly coat foreign matters on the electrode surface in even thickness, thereby drastically improving electric short-circuiting characteristics of an electronic device being fabricated. In this regard, the wet process is more advantageous and higher in superiority.
Hence, we made continued studies on the fabrication of a charge transporting thin film responsible for electronic devices, to which a wet process is applicable (see, for example, Patent Document 2).
The wet process needs a very large amount of a charge transporting varnish necessary for forming a charge transporting thin film, for example, by a spin coating process or printing process. More particularly, aside from a charge transporting varnish contributing to the formation of a charge transporting thin film, a ratio of a charge transporting varnish to be wasted becomes very large, resulting in poor cost performance.
For a process of making a charge transporting thin film in high efficiency, a spray process or ink jet process can be mentioned. Particularly, the spray process is effective, from the standpoint of the general versatility of apparatus, as a wet process capable of making a large-area device.
Consequently, it becomes necessary to provide a charge transporting varnish that is optimum for forming, according to a spray process or an ink jet process, a uniform, unevenness-free charge transporting thin film on an exposed ITO or IZO electrode portion of a substrate forming a structure of a simple or complicated pattern thereon.
This problem of forming a charge transporting thin film on a substrate forming a structure of a simple or complicated pattern thereon is important not only for the thin film for electronic devices, but also for other technical fields using thin films.
For the reasons set out above, there is a demand for development of a charge transporting varnish suited for the spray or ink jet process.
Patent Document 1:                JP-A 2002-151272        
Patent Document 2:                PCT Patent Publication No. WO 2004/043117        